Referring to FIG. 1, which is a perspective view of a steer axle assembly according to the present invention, an axle main beam 15, which is a load bearing structure, has offset ends 24 which extend forward from the axle main beam. Stub axles 20 are pivotably mounted to the ends 24 by machined kingpin assemblies 22. The offset that exists between the axis of rotation of the stub axle assembly and the axis of the axle main beam forms the basic geometric character of all leading kingpin self-steering axles. A greater offset leads to an increased sensitivity of the axle steering function.
A tie rod 14 connects both pivot ends together and harmonises the wheels' steering behaviour. The axle is mounted with the leading kingpins facing the forward direction of the truck or trailer. Tie rod assemblies can be mounted forward or rearward of the axle beam depending upon the geometry of the suspension being used.
Self-steering axles tend to return naturally to a neutral (straight forward) position once the vehicle returns to its normal forward direction after making a turn. A stabilizer system consisting of a resilient member in the form of a double convolute air spring 12 is attached to the main axle beam and the tie rod assembly and has mechanical pivoting connections (lever arms 7 and 8) to allow the air spring 12 to constantly apply a resistance to a respective spring ends 17 and 18 of each of the lever arms 7 and 8 against the steering motion of the axle, and help the axle return to the normal straight ahead position after the turn.
The double convolute air spring 12 receives a constant regulated supply of air pressure from the main trailer/truck system.
Axle camber is pre-set at the factory during the production process. Caster of the axle is set when the axle is assembled to the suspension and mounted to the trailer or truck. The manufacturer of the axle supplies definite numbers for the setting of the caster angle which is normally 0 degrees to 3.0 degrees positive which is set to the trailer manufacturer's specified ride height and that is designed to have the trailer use a specified coupler height. However, in today's transportation scene there is a multiplicity of fifth wheel mounting heights and trailer coupling heights and also, a great variance in the suspensions being used. The positioning of the axle on the trailer also plays an important role in the performance of the steering axle. The closer the axle is to the front of the trailer the more it will be affected by the variance in coupler heights. For example, connecting a 48″ coupler height trailer to a 51″ truck fifth wheel height will change the caster to 3-5 degrees depending upon the position of the axle on the trailer. This is an out of specification situation.
Such changes in caster angle will affect steering performance, increase tire wear, and introduce wheel hop and shimmy into the steering axle. The double convolute air spring system is not sensitive enough to be able to adjust to the changes in the caster angle, therefore, it cannot control out of specification situations. This problem is apparent in all leading kingpin design steering axles using air springs as their main source of stabilization. This problem is intensified when the axle is closer to the tractor.